1. Field of the Invention
The present invention relates to an illumination optical system, exposure apparatus, and device manufacturing method.
2. Description of the Related Art
Generally when performing an exposure process, the illumination range of a reticle is restricted by an illumination field stop formed in an illumination optical system so that no light hits an unwanted range of a reticle or mask serving as a circuit original. However, when the illumination range is controlled to execute an exposure process, the image of the illumination field stop blurs owing to aberration of an optical system which forms the image of the illumination field stop onto a reticle. As a result, the exposure amount decreases near a light shielding portion. The decrease in exposure amount is therefore prevented by offsetting the illumination field stop in the opening direction with respect to a target illumination range. Japanese Patent Laid-Open No. 2000-252193 discloses a technique of setting a light shielding band around a reticle pattern and widening the illumination field stop in the opening direction by ½ of the light shielding band width.
In a conventional exposure apparatus, the illumination field stop is widened in the opening direction by a predetermined offset amount regardless of the illumination range. However, the blur amount changes or the image forming position shifts depending on the illumination range owing to aberration of the optical system which forms the image of the illumination field stop onto a reticle. If the illumination range of the illumination field stop is changed to perform exposure, the offset amount becomes short, an image blurs in a predetermined illumination range, and the exposure amount decreases.